1. Field of the Invention
This invention relates to a piezoelectric ceramic composition. More particularly, it relates to a piezoelectric ceramic composition which has improved dielectric and piezoelectric characteristics.
2. Description of the Prior Art
Barium titanate type ceramic compositions and lead zirconatetitanate type ceramic compositions have been used as piezoelectric ceramic compositions. These compositions, however, are not stable industrial compositions. Therefore, efforts have been directed to developing improved piezoelectric compositions. One of these improved compositions is a ternary system of a solid solution of Pb(Mg.sub.1/3.Nb.sub.2/3)O.sub.3 -PbTiO.sub.3 -PbZrO.sub.3. It has also been proposed to modify and improve the piezoelectric properties of the composition by the addition of an oxide of Mn, Cr, Co, Fe or Ni to the basic composition. One of the problems with this composition is the moisture absorbing property of the composition containing Mg which interfers with accurate weighing of the composition and the storage of the starting materials. Although the composition has an electro-mechanical coupling coefficient, Kp of 50-58%, the dielectric loss of the composition is as high as 2-2.4%.
In the ternary solid solution of the base ceramic components of PbTiO.sub.3 -PbZrO.sub.3 and the additive, Pb(Mn.sub.1/3 Sb.sub.2/3)O.sub.3, the dielectric constant .epsilon. and the electro-mechanical coupling coefficient Kp are easily changed by small changes in the components. Compositions having Kp values greater than 50% have low dielectric constants of about 500. Since compositions having a dielectric constant greater than 1000 have been limited to rather narrow ranges of the components, the number of industrially suitable piezoelectric has been limited to a relative few. The compositions of the prior art have been combinations of the above base ceramics components and a perovskite type component, ABO.sub.3.
Other proposed improvements for the compositions have involved the addition of certain additives to the base ceramic components of PbTiO.sub.3 -PbZrO.sub.3. For example, in one procedure, the basic ceramic composition has been modified by substituting Pb by Sr and by the addition of a component of Sb.sub.2 O.sub.3 and MnO.sub.2. Up to the present, the most effective modified piezoelectric composition has been one whose base ceramic components of PbTiO.sub.3 and PbZrO.sub.3 are in a ratio of 47:53 and which has had values of Kp=60%, .epsilon.=1750 and tan .delta.=2.5%. However, these compositions have had the disadvantage that their performance characteristics are readily changed by the small changes in their component structure. For example, a change in the quantity of Mn of 0.15 wt.% causes a change of Qm from about 1170 to about 300. If it is desired to decrease the tan .delta. value to 1.0% by adding another component, the value of Kp changes from 60% to 48.3%. To the present, piezoelectric ceramic compositions having the desired combination of Kp, .epsilon., Q and tan .delta. values which are stable to small changes in the component makeup of the composition have not been obtained.
A need therefore, continues to exist for piezoelectric compositions which have Kp, .epsilon., Q and tan .delta. values which are stable to small changes in the component makeup of the compositions, and which have high piezoelectric constants.